Format: Modernize using variadic templates.

Introduces a subset of C++14 integer sequences in STLExtras. This is
just enough to support unpacking a std::tuple into the arguments of
snprintf, we can add more of it when it's actually needed.

Also removes an ancient macro hack that leaks a macro into the global
namespace. Clean up users that made use of the convenient hack.

llvm-svn: 229337